1. Field of the Invention
The invention is directed to high solids vinyl acetate-maleic anhydride copolymer latex coating compositions having uniformly small particle size and improved starch compatibility.
2. Description of the Prior Art
Heretofore, copolymers of vinyl acetate and derivatives of alpha, beta-unsaturated dicarboxylic acids such as maleic anhydride, were polymerized in solution using an organic solvent such as ethyl acetate, ethyl alcohol, benzene or acetone and a free radical initiator such as benzoyl peroxide; see U.S. Pat. No. 2,637,712. Because of the use of such solution polymerized copolymers in coating compositions, one has had to employ varying techniques to achieve an effective composition. One such example is set forth in U.S. Pat. No. 3,799,902 in which a vinyl acetate-maleic anhydride copolymer is emulsified in the form of a water-in-oil emulsion. This technique requires the time-consuming step of grinding the copolymer to the desired particle size, e.g. in a range of 1-5 microns, and dispersing the particles in the water-in-oil emulsion. Typical solvents include benzene, xylene, toluene, mineral oils, kerosene and naphthas. The emulsification is done in the presence of a suitable emulsifying agent such as sorbitan monooleate.
Another example of a coating composition which encompasses vinyl acetate-maleic anhydride copolymers is found in U.S. Pat. No. 3,632,546. Specifically, a vinyl acetate-maleic anhydride copolymer is illustrated as being produced by solution polymerization, bulk polymerization, suspension polymerization or emulsion polymerization using a non-aqueous solvent. There is no suggestion in this reference of preparing such copolymers using aqueous emulsion polymerization techniques.
It is known to employ a copolymer vinyl acetate and maleic acid, not maleic anhydride, by polymerizing in an aqueous media in coating compositions; see U.S. Pat. No. 3,053,785. However, coating compositions using such a copolymer have not been found to have improved starch binder compatibility.